A diver has to breathe compressed air or a mixture of gases containing the appropriate amount of oxygen at a pressure equal to the hydrostatic pressure compressing the thorax, in order to be able to inflate the lungs. As the gas pressure is increased with the depth to which he descends, the quantity of gases physically dissolved in the blood plasma and the body tissues increases proportionally. While oxygen is metabolized, all other gases in the mixture will come out of solution again as soon as the hydrostatic pressure on the diver is reduced during his ascent to the surface. This decompression has to proceed at a very slow rate in order to prevent the occurrence of the phenomena seen when a bottle of a sparkling liquid is opened and its contents are abruptly decompressed: the dissolved gases are immediately released and coalesce to form bubbles. The gases that are not metabolized by the organism must be prevented from coming out of solution in this way since even small bubbles could block peripheral capillaries and thereby increase the load on the heart. In more serious cases, the gases form larger bubbles which can produce gas embolisms in the blood vessels of vital organs, infarctions and necroses of the surrounding tissues and will ultimately lead to death.
The clinical signs of decompression sickness following a too rapid ascent to the surface correspond to the accumulation of gas bubbles in the joints. The person so afflicted minimizes the load on his/her articulations and maintains a distorted or bent posture, thus `the bends`. The later stages of decompression sickness are characterized by increasingly severe neurological disorders which are explained with functional losses in the central nervous system due to air embolisms.
Decompression sickness is prevented by requiring divers to follow a strict ascent schedule which takes into account the depth reached and the duration of stay at depth. Ascent tables have been established semi-empirically and are constantly revised as the knowledge of the various physical and physiological parameters determining the distribution of gases in blood and the tissues as a function of muscular activity increases. Replacing nitrogen in the respired air with helium, a gas that has a much lower solubility in the tissues, has somewhat reduced the incidence of decompression sickness but an accident at work might require an ascent too rapid to prevent the bends even under such a "Heliox" (trademark) atmosphere.
Divers brought to the surface too rapidly will always suffer from the bends to individually varying degrees. They will, in most cases, recuperate completely, i.e. without any apparent permanent lesions, if brought promptly into a recompression chamber where barometric pressure corresponding to the hydrostatic pressure at depth can be established and in which the slow ascent to the surface is simulated. Recompression chambers are very costly and are mainly found on land; only a few research vessels for deep-sea diving are so equipped. They are thus invariably located at considerable distances from the scene of a dive and the consequent delay in treatment increases the risk to the diver suffering from the symptoms of decompression sickness. Several symptomatic treatment schemes are available but they are only supplementary to recompression.
The deep-sea exploration for petroleum and the mining of minerals bring divers even further away from the few centers where a prompt and complete treatment of decompression sickness can be effected. Emergency recompression chambers found on off-shore drilling rigs, for example, are small mobile units in which a diver can be evacuated but in which he cannot be treated. Some accidents in underwater work require immediate medical attention and do not leave time for the elaborate ascent schedule that can last several hours or even several days. Incidents have been reported in which recompression treatment was delayed by hours with, subsequently, the diver's complete recovery, but the individual variations are great and these cases cannot be used to establish a therapeutic strategy. Decompression by bringing the diver back into the water is not meant to replace treatment in a chamber. At present, divers are all too often confronted with the alternative to succumb to an injury at depth subsequent to only a minor working accident, or, to suffer the consequence of a rapid decompression, with a real possibility to die from it.
The sudden failure of a pilot's pressurization equipment at altitude (explosive decompression) leads to the same phenomena and produces the same results.